Process for converting ammonium imidodisulfate, ammonium sulfamate and ammonium dithionate to sulfur dioxide and ammonia

ABSTRACT

Ammonium imidodisulfate and ammonium sulfamate may be converted to sulfur dioxide and ammonia by heating them with a reducing agent or a catalyst in a bath of molten ammonium sulfate and ammonium bisulfate. The reducing agent is sulfur or a sulfur compound and the catalyst is selected from Mo, W, Cu, Fe, Co, Cr, Mn and Ni.

The invention concerns a process for converting ammonium imidodisulfate (IMDS) and/or ammonium sulfamate (SMA), in the optional presence of ammonium dithionate (DTN), to sulfur dioxide and ammonia.

These salts are recovered, for example, from the simultaneous absorption of nitrogen oxides and sulfur dioxide present in combustion gas, with aqueous solutions comprising ammonium sulfite and an iron chelate, for example the iron chelate of ethylenediaminotetracetic acid. These salts have no important use and their discharge represents an important loss of reactants and creates a pollution problem.

It has been already proposed to convert these salts to ammonium sulfate by oxidation and hydrolysis, but the latter material has no substantial market.

It would be more beneficial to recover ammonia from these salts and to recycle it to the step of SO₂ and nitrogen oxide absorption.

It has been found that these salts can be converted to a gas mixture comprising SO₂ and NH₃ by contacting them with a molten mixture of ammonium bisulfate and sulfate.

The SO₂ and NH₃ containing gas mixture may be treated thereafter by any known means to recover, separately or in admixture, the constituents in the form of NH₃, SO₂ or sulfur.

The process comprises contacting at least one of these salts with molten ammonium sulfate and/or bisulfate, at a temperature of 280° to 420° C., in the presence of a sulfur-containing reducing agent, such as sulfur, hydrogen sulfide, ammonium thiosulfate and/or ammonium polysulfide, and/or a catalyst comprising Mo, W, Cu, Fe, Co, Cr, Mn and/or Ni.

In the selected temperature range, equilibrium between ammonium sulfate and ammonium bisulfate occurs. It is thus possible, without disadvantage, to prepare the bath from ammonium sulfate, ammonium bisulfate or a mixture of both.

In the presence of the above reducing agents, the decomposition of the above salts takes place mainly according to the following reactions: ##STR1##

It is easy to deduce the stoichiometrical proportion from the above relations, when using an ammonium polysulfide.

It has thus been found that sulfur and nitrogen of these salts are converted respectively to SO₂ and NH₃.

When the ammonium sulfate/bisulfate bath contains a Mo, W, Cu, Fe, Co, Cr, Mn and/or Ni salt, in the absence of reducing agents, the salt decomposition takes place essentially according to the reactions: ##STR2##

It is easy to deduce the stoichiometrical proportion from the above relations, when operating with an ammonium polysufide.

In that case, nitrogen of the salts is converted to molecular nitrogen and ammonia, while sulfur is converted to SO₂.

When operating in the simultaneous presence of a reducing agent and a catalyst, the salt decomposition takes place according to several of the above reactions in variable proportions depending on the temperature. As a rule, the reactions of reduction with S, H₂ S and (NH₄)₂ S₂ O₃ are preponderant at the lower temperatures, about 280° to 350° C., while the decomposition reactions prevail at temperatures above 350° C. It has also been found that the above catalysts substantially increase the reduction rate, particularly when the reduction agent is H₂ S.

The process of the invention may be operated at a temperature of from 280° to 420° C., preferably 300° to 380° C. When operating only with a catalyst, it is desirable to proceed in the temperature range of from 350° to 380° C., thereby obtaining high reaction rates. At temperatures lower than 280° C., the reaction rates are low and ammonia is retained in the bath of molten salt by ammonium bisulfate which is converted to ammonium sulfate, which may result in the solidification of the bath. At temperatures above 420° C., ammonium bisulfate decomposes at a too high rate, so that the bath volatilizes.

The process is usually operated at a pressure close to atmospheric pressure. A higher pressure may be used, if necessary; however too high pressures may inhibit the ammonia release and result in solidification of the bath. The useful pressure range is from 0.5 to 10 atm., preferably 1 to 5 atm.

The process is usually operated by injecting the salts, either individually or in admixture, as such or in aqueous solution, into a reactor containing an ammonium sulfate/bisulfate bath at a temperature of 280° to 420° C.

The charge of salts may also comprise, without disadvantage, other ammonium salts, such as ammonium sulfites and sulfates. By adjusting the operating conditions, they are also decomposed to SO₂ and NH₃.

When using a catalyst, the latter may be introduced into the bath as powdered metal, oxide, sulfide or salt such as, for example, Mo, W, Cu, Fe, Co, Cr, Mn and/or Ni sulfate, nitrate or acetate, in a preferred proportion of 0.1 to 10% b.w. of metal. Continuous supply of catalyst is not necessary.

When operating without catalyst, the reducing agent is supplied continuously or by fractions in admixture with the feed charge or separately in at least stoichiometrical reaction proportions. When using both a reducing agent and a catalyst, the proportion of reducing agent may be reduced, if desired.

Temporary accumulation of the salts to be decomposed, in the ammonium sulfate/bisulfate bath, due for example to a temperature decrease or a too low proportion of reducing agent, does not alter the overall running of the plant. It results in an increase of the bath volume, which can be brought back to its normal value by adjusting the operating conditions. The temporary use of an excess of reducing agent or a too high temperature may result in a decrease of the bath volume due to ammonium sulfate decomposition. The normal volume of the bath may be attained by decreasing the temperature and/or the feed rate of the reducing agent.

The gas discharged from the reactor comprises SO₂ and NH₃, possibly also H₂ O and/or N₂ and, from time to time, low amounts of H₂ S and S from the unconverted reducing agents. When the reactor is heated by direct contact of the salt bath with hot gas from a burner, the latter gas is also present in the effluent gas.

The gas mixture may be treated thereafter by any known method to recover, preferably separately, NH₃ and a valuable sulfur compound such as So₂, H₂ SO₄ or even sulfur. Reference is made to the process of the French Pat. No. 2,199,475 according to which the SO₂ and NH₃ containing gas mixture is admixed with a H₂ and/or CO containing reducing gas and contacted first with a solid catalyst, so as to convert a portion of SO₂ to H₂ S, and then a selective solvent to convert therein H₂ S and SO₂ to elemental sulfur. Ammonia may be recovered thereafter.

The following, non limitative examples, illustrate the invention:

EXAMPLE 1

400 g of ammonium hydrogen sulfate are introduced into an electrically heated quartz reactor which is thereafter heated to 350° C. 50 cc/hour of an aqueous solution of 1 mole/liter ammonium imidodisulfate and 2 moles/liter ammonium dithionate is pumped into the bath of molten material. 7 g/hour of liquid sulfur is also pumped into the bath. Stirring of the bath is obtained by bubbling therethrough 100 liters of nitrogen per hour and the temperature is maintained at 350° C.

Ammonia is analysed in the effluent gas from the reactor by absorption in a sulfuric acid solution which is thereafter analysed back with a titrated sodium hydroxide solution. So₂ is analysed by absorption in a sodium hydroxide solution which is titrated with iodine.

The analysis of the gas gives the following results with respect to time.

                  TABLE I                                                          ______________________________________                                                     SO.sub.2 flow rate                                                                           NH.sub.3 flow rate                                   Time in hours                                                                              moles/hour    moles/hour                                           ______________________________________                                         1           0.385         0.330                                                2           0.410         0.355                                                4           0.390         0.352                                                ______________________________________                                    

The theoretical flow rates of SO₂ and NH₃ corresponding to the sulfur reduction of ammonium imidodisulfate and dithionate are respectively 0.4 and 0.35 mole/hour.

The level of the ammonium sulfate bath does not vary in the course of time and the analysis of the bath at the end of the experiment shows that no accumulation of IMDS and DTN has taken place.

It is thus found that the decomposition of IMDS and DTN to SO₂ and NH₃ is practically quantitative.

EXAMPLE 2

Example 1 is repeated, except that sulfur is replaced with other reducing agents or a catalyst is introduced into the sulfate bath. The following table summarizes the operating conditions and the average SO₂ and NH₃ hourly feed rates, as determined in the effluent gas for 4 hour-experiments. The theoretical rates, also stated, relate to the reduction of the salts, when using reducing agents, or to their decomposition, when using a catalyst (according to the above stated reactions).

The results of experiments 1 to 3 show that, in the selected operating conditions, the imidodisulfate and the dithionate are effectively reduced by the reducing compounds: S, (NH₄)₂ S₂ O₃ and H₂ S.

The results of experiments 4 to 7 relate to the embodiment of the process which is operated in the presence of a catalyst and without reducing agent. They show that the quantitative decomposition of ammonium imidodisulfate and dithionate may be obtained without decomposing the ammonium sulfate of the bath by judiciously selecting the operating temperature.

The results of experiments 8 to 10 show that the simultaneous use of a catalyst (Cu) and a reducing agent (H₂ S) increases the imidodisulfate reduction rate and results in partial reduction of the ammonium bisulfate of the bath. A reduction of the H₂ S feed rate suppresses the parasitic reduction of ammonium bisulfate, while maintaining the quantitative reduction of imidodisulfate.

                                      TABLE II                                     __________________________________________________________________________     Salt Injection                                                                 (moles/h)                                                                      Expe-                                                                              Ammon-                                                                               Ammon-                     SO.sub.2 Feed Rate                                                                      NH.sub.3 Feed Rate               ri- ium   ium                        moles/h  moles/h                          ment                                                                               imidodi-                                                                             dithio-                T bath                                                                             Theore-                                                                             Mea-                                                                               Theore-                                                                             Mea-                        No. sulfate                                                                              nate Reducing Agent                                                                             Catalyst                                                                             °C.                                                                         tical                                                                               sured                                                                              tical                                                                               sured                                                                              Remarks                 __________________________________________________________________________     1   0.05  0.1  Sulfur 7 g/h                                                                               None  350 0.4  0.395                                                                              0.35 0.345                                                                              The effluent                                                                   contains sulfur.        2   0.05  0.1  (NH.sub.4).sub.2 S.sub.2 O.sub.3                                                      25 g/h                                                                              None  350 0.569.sup.(1)                                                                       0.585                                                                              0.688.sup.(1)                                                                       0.680                                                                              The effluent                                                                   contains sulfur.        3   0.05  0.1  H.sub.2 S                                                                             3 Nl/h                                                                              None  350 0.367                                                                               0.35                                                                               0.35 0.33                                                                               The effluent                                                                   contains H.sub.2                                                               S.                      4   0.05  0.1  None        CoSO.sub.4 -20 g                                                                     380 0.300                                                                               0.285                                                                              0.217                                                                               0.195                       5   0.05  0.1  None        Mo.sub.2 O.sub.3 -20 g                                                               365 0.300                                                                               0.310                                                                              0.217                                                                               0.225                       6   0.05  0.1  None        Mo.sub.2 O.sub.3 -20 g                                                               380 0.300                                                                               0.450                                                                              0.217                                                                               0.275                                                                              the volume                                                                     of the bath                                                                    decreases.              7   0.05  0.1  None        Mo.sub.2 O.sub.3 -20 g                                                               340 0.300                                                                               0.180                                                                              0.217                                                                               0.174                                                                              The volume                                                                     of the bath                                                                    increases.              8   0.15  0    H.sub.2 S                                                                             5 Nl/h                                                                              None  345 0.400.sup.(2)                                                                       0.415                                                                              0.450.sup.(2)                                                                       0.455                       9   0.15  0    H.sub.2 S                                                                             5 Nl/h                                                                              CuSO.sub.4 -25 g                                                                     345 0.400.sup.(2)                                                                       0.650                                                                              0.450.sup.(2)                                                                       0.510                                                                              The volume                                                                     of the bath                                                                    decreases.              10  0.15  0    H.sub.2 S                                                                             2.4 Nl/h                                                                            CuSO.sub.4 -25 g                                                                     345 0.400.sup.(2)                                                                       0.405                                                                              0.450.sup.(2)                                                                       0.420                       11  0     0.2  S      7 g/h                                                                               None  355 0.500                                                                               0.510                                                                              0.400                                                                               0.395                       __________________________________________________________________________      .sup.(1) The excess of (NH.sub.4).sub.2 S.sub.2 O.sub.3 with respect to        the stoichiometrical proportion for reducing the imidodisulfate and the        dithionate is supposed to decompose according to the reaction :                (NH.sub.4).sub.2 S.sub.2 O.sub.3 →S + SO.sub.2 + 2 NH.sub.3 +           H.sub.2 O.                                                                     .sup.(2) According to the reaction of H.sub.2 S with ammonium                  imidodisulfate and dithionate.                                           

EXAMPLE 3

400 g of ammonium hydrogen sulfate are introduced into an electrically heated quartz reactor which is thereafter heated to 350° C. 50 cc/hour of an aqueous solution of 2 moles/liter of ammonium sulfamate is pumped into the bath. 3.5 g/hour of liquid sulfur is also pumped thereinto. Stirring is obtained by bubbling 100 liters nitrogen per hour through the bath and the temperature is maintained at 350° C.

The effluent gas has been analysed in the course of time and the following results have been obtained:

                  TABLE III                                                        ______________________________________                                                     SO.sub.2 flow rate                                                                           NH.sub.3 flow rate                                   Time in hours                                                                              moles/hour    moles/hour                                           ______________________________________                                         1           0.130         0.175                                                2           0.152         0.195                                                4           0.145         0.210                                                ______________________________________                                    

The theoretical SO₂ and NH₃ flow rates corresponding to the sulfur reduction of the injected ammonium sulfamate are respectively 0.15 and 0.2 mole/hour.

The level of the ammonium sulfate bath does not vary in the course of time and the analysis thereof, at the end of the experiment, shows that no sulfamate has accumulated.

EXAMPLE 4

Example 3 is repeated, except that sulfur is replaced with other reducing agents, or a catalyst is introduced into the sulfate bath. The following Table summarizes the operating conditions and the average flow rates of SO₂ and NH₃ in the effluent gas over 4 hours.

The theoretical rates, as shown in the Table, relate to the reduction of the same salts, when using reducing agents, or to their decomposition, when using a catalyst (according to the above reactions).

The results of experiments 1 to 3 show that, in the above operating conditions, sulfamate is effectively reduced by the reducing compounds: S, (NH₄)₂ S₂ O₃ and H₂ S.

Experiments No. 4 and 5 show that the quantitative decomposition of the sulfamate may also be obtained without reducing agent, in the presence of a catalyst.

                                      TABLE IV                                     __________________________________________________________________________     Expe-                                                                              Ammonium                     SO.sub.2 Feed Rate                                                                        NH.sub.3 Feed Rate                 ri- Sulfamate                    moles/h    moles/h                            ment                                                                               Injected                 T bath                                                                             Theore-    Theore-                            No. (moles/h)                                                                            Reducing Agent                                                                             Catalyst                                                                              °C.                                                                         tical                                                                               Measured                                                                             tical                                                                               Measured                                                                             Remarks                 __________________________________________________________________________     1   0.1   sulfur 3.5 g/h                                                                             None   350 0.15 0.144 0.2  0.197 The effluent                                                                   contains sulfur.        2   0.15  (NH.sub.4).sub.2 S.sub.2 O.sub.3 20 g/h                                                    None   350 0.375.sup.(1)                                                                       0.38  0.6.sup.(1)                                                                         0.58  The effluent                                                                   contains sulfur.        3   0.15  H.sub.2 S 2.3 Nl/h                                                                         None   345 0.2  0.19  0.3  0.29  The effluent                                                                   contains H.sub.2                                                               S                       4   0.15  None        Mo.sub.2 O.sub.3 20 g                                                                 370 0.15 0.16  0.2  0.21                          5   0.15  None        CuSO.sub.4 25 g                                                                       380 0.15 0.14  0.2  0.2                           6   0.15  H.sub.2 S 1.4 Nl/h                                                                         CuSO.sub.4 25 g                                                                       345 0.2.sup.(2)                                                                         0.21  0.3.sup.(2)                                                                         0.31                          __________________________________________________________________________      .sup.(1) The excess of (NH.sub.4).sub.2 S.sub.2 O.sub.3 with respect to        the stoichiometrical proportion for reducing ammonium sulfamate is             supposed to decompose according to the reaction: (NH.sub.4).sub.2 S.sub.2      O.sub.3 →S + SO.sub.2 + 2 NH.sub.3 + H.sub.2 0.                         .sup.(2) According to the reaction of H.sub.2 S with ammonium sulfamate.  

What we claim is:
 1. A process for converting ammonium imidodisulfate and/or ammonium sulfamate to sulfur dioxide and ammonia, wherein the said ammonium salt(s) is (are) contacted with a molten mixture of ammonium bisulfate and ammonium sulfate, at a temperature from 280° to 420° C., in the presence of a sulfur-containing reducing agent and/or a catalyst comprising at least one member of the group consisting of molybdenum, tungsten, copper, iron, cobalt, chromium, manganese and nickel, so as to obtain an effluent gas comprising sulfur dioxide and ammonia.
 2. A process according to claim 1, wherein ammonium dithionate is also present.
 3. A process according to claim 1, operated at a pressure from 0.5 to 10 atm.
 4. A process according to claim 1, operated at a pressure from 1 to 5 atm.
 5. A process according to claim 1, wherein the amount of reducing agent supplied to the molten mixture of ammonium bisulfate and ammonium sulfate, in the absence of catalyst, is at least the stoichiometrical proportion corresponding to the decomposition of ammonium imidodisulfate, ammonium sulfamate and/or ammonium dithionate injected during the same time.
 6. A process according to claim 1, wherein the catalyst is present in the bath of ammonium bisulfate and ammonium sulfate in a proportion of 0.1 to 10% b.w., expressed as metal, of the weight of the bath.
 7. A process according to claim 1, wherein the sulfur-containing reducing agent is sulfur, hydrogen sulfide, ammonium thiosulfate or an ammonium polysulfide.
 8. A process according to claim 1, wherein the ammonium imidodisulfate and the ammonium sulfamate are introduced in the form of the absorption product of a mixture of sulfur dioxide and nitrogen oxide by an aqueous solution of ammonium sulfite and iron chelate. 